Ohm's law calculator
Leave the unknown blank. Enter the other two. This is V = IR for a resistive element, not an AC impedance solve.
Method
V = IR. Power = VI. Worked case: 230 V across 46 Ω draws 5 A and dissipates 1,150 W.
Do not use this for cable sizing, motor starting or anything with reactance.
What to write beside the result
A review can only check this figure if the inputs sit next to it. Write the value, the unit and the assumption the form used. A diameter in millimetres and a diameter in metres are not the same reading. A force in kilonewtons divided as if it were newtons is a thousand-fold error.
The result is the arithmetic on this page. It is not a utilisation, not a selected product, and not a clause from a standard. If the number is going into a shared container, name the page, the date and the inputs. The file-name checker does not read the calculation.
Logic: how this figure is built.
How to read Ohm's law
This page rearranges V = IR. Two of voltage, current and resistance give the third. It is the direct-current, or the sinusoidal-rms, relationship for a resistive load. It is not an impedance calculation.
A 12 V supply across 6 Ω draws 2 A and dissipates 24 W if the resistor can take it. The page gives the current. It does not give the resistor rating. A coil, a capacitor and a motor are not a single resistance at every frequency.
Mixing peak and rms values is the usual error on an alternating supply. Nameplate voltages are rms. A peak voltage used as if it were rms overstates the current.
Use it to check a measured pair against the third quantity. Cable sizing and disconnection times are a different calculation.
- Related: voltage drop
- electrical logic
Last checked 3 October 2026. Worked figures are scenarios. See methodology and the disclaimer.